The present invention relates to a gas-blast switch.
Such a gas-blast switch is already known from U.S. Pat. No. 4,139,732. In this switch, heated and compressed quenching gas is stored in a heating volume after the switching arc is switched off. During the switching off operation, compressed quenching gas is additionally generated by the movement of a compressor piston in a compression space joined to the heating volume via a back-pressure valve. This measure makes it possible to achieve a quenching of the switching arc with a comparatively low drive energy even when the pressure of the quenching gas, compressed by heating, is low in the heating volume. When high short-circuit currents are switched, the high pressure of the compressed quenching gas stored in the heating volume, however, closes the back-pressure valve and impedes the movement of the compression piston sliding in the compression volume.
European Patent Specification No. 0,035,581, describes a gas-blast switch in which the blast pressure needed for quenching the arc is generated by the arc itself in an arc space and carried via a back-pressure valve into a pressure storage space where it is stored until the blasting starts. In addition, a compression piston is specified which makes available, with a time delay, further compressed quenching gas which is fed via a back-pressure valve into the pressure storage space and from there flows together with the compressed quenching gas generated by the arc itself via a further back-pressure valve into the arc space and from there flows into an expansion space, blasting the arc.
With small switching-off currents, the blasting pressure generated by the arc is low so that the arc space is sufficient for storing it and the back-pressure valve does not open from the arc space to the pressure storage spaced. The compressed quenching gas generated by the compression piston, independently of the magnitude of the switching-off current, flows as previously mentioned through two further back-pressure valves located behind each other into the arc space and from there blasts the arc.
With very large switching-off currents, the gas pressure generated by the arc is so large that both the arc space and the pressure storage space are fully charged by it. Initially, the compressed quenching gas generated by the compression piston can not therefore pass into the pressure storage space since it is at a lower pressure. As a function of stroke the pressure in the compression volume under the compression piston rises further since no release into the pressure storage space can take place. To avoid overloading the switch drive, a further adjustable back-pressure valve is mounted at the compression housing which vents the compression volume directly into the expansion space if the compression pressure is too high. The numerous back-pressure valves, which are partially located in an area which can be reached by hot gases and switching residues, must be considered as weak points since spring defects or erosion of the sealing seats cannot be prevented at these locations.
The invention as characterised in the claims has the objective of creating a gas-blast switch of the just above mentioned type, the switching-off capacity of which is increased with simultaneous reduction of the drive energy.
The gas-blast switch according to the invention is characterised by the fact that the compression pressure is limited or reduced selectively and in a reliable manner before a certain maximum value determining for the drive design is exceeded, without using additional moving parts, and that this at the same time opens the possibility for a particularly effective dual blasting of the switching arc to be quenched. It is also found to be advantageous that the compression slide valve forces are controlled during the entire switching-off stroke in such a manner that excessive stresses are avoided.